Preparation of oxalic acid

ABSTRACT

An oxalate salt from which oxalic acid may be produced is prepared by the cationic reduction of carbon dioxide in an electrolytic cell wherein the anode and cathode compartments are separated by a porous membrane and the catholyte is an organic solvent. Tetraethylammonium perchlorate, tetraethylammonium bromide, tetrabutylammonium perchlorate, tetrabutylammonium iodide and tetraethylammonium p-toluenesulfonate are the preferred solutes for the catholyte. Coulombic yields as high as 75 percent are obtained where the anolyte is the same electrolyte and solvent as the catholyte while yields as high as 97 percent of sodium oxalate are obtained when aqueous solutions of sodium salts are used as the anolyte.

United States Patent Skarlos 1March 13, 1973 PREPARATION OF OXALIC ACID PrimaryExaminer-F. C. Edmundson [75] inventor: Leonidas Skarlos, Richmond, Va. Attorney-Thomas whaley [73] Assignee: Texaco Inc., New York, N.Y. [57] ABSTRACT [22] Filed: Dec. 28, 1971 An oxalate salt from which oxalic acid may be produced is prepared by the cationic reduction of car- [211 Appl' 2l3206 bon dioxide in an electrolytic cell wherein the anode and cathode compartments are separated by a porous 521 vs. Cl. ..204 59 R, 260/538 membrane and the Catholyte is an Organic Solvent- [511 Int 29/06, C076 51/40, C07c 55/06 Tetraethylammonium perchlorate, tetraethylammoni- [58] Field of Search ..204/59, 72- 260/538 bmmide tetrabulylammmium Perchlmte, tetrabutylammonium iodide and tetraethylammonium 5 f p-toluenesulfonate are the preferred solutes for the 6] Re erences catholyte. Coulombic yields as high as 75 percent are UNITED STATES PATENTS obtained where the anolyte is the same electrolyte and solvent as the catholyte while yields as high as 97 per- 3,032,489 5/1962 Loveland ..204/59 X cent of sodium oxalate are obtained when aqueous 3,393,136 7/]968 Tenton et al. ..204/59 solutions of Sodium Salts are used as the y 3,344,045 9/1967 Nelkam ..204/59 10 Claims, No Drawings 1 PREPARATION or OXALIC ACID BACKGROUND OF THE INVENTION This invention relates to the preparation of oxalic acid. More particularly, this invention relates to the preparation of an oxalate salt by an electrolytic process. The oxalate salt may be recovered from the electrolytic cell and acidified to convert it to oxalic acid.

Oxalic acid and oxalate salts are produced commercially to fill a great variety of end uses. For example, they are used in laundries as a rust and ink remover, as the chief constituent in automobile radiator scale removers, as an electrolyte in the anodic oxidation of aluminum, as a bleaching agent for such materials as straw, cork, rosin and wood, as reagents in chemical analysis and in the manufacture of miscellaneous chemical derivatives. Oxalic acid is prepared commercially by four general methods:

(1) the fusion of wood waste with alkali metal salts to produce sodium oxalate, (2) the oxidation of carbohydrates (sugar, starch or cellulose) with concentrated nitric acid in the presence of a vanadium catalyst to yield oxalic acid, (3) the fermentation of sugar by molds and (4) the catalytic conversion of sodium formate to sodium oxalate which is converted to oxalic acid. The major portion of oxalic acid production is by the latter method in which sodium formate is prepared synthetically from sodium hydroxide and carbon monoxide. In this process the conversion of formate to oxalate requires critical control and the entire process efficiency depends upon the success of this particular operation. The presence of impurities, the heating rate and the temperature are all critical factors in this conversion.

A procedure to prepare oxalic acid by means not currently employed commercially which may offer advantages over known processes would be highly desirable and is an objective of this invention.

SUMMARY OF THE INVENTION By means of my invention, an oxalate salt capable of being converted to form oxalic acid is prepared by an electrolytic technique. My invention relates to the cationic reduction of carbon dioxide in an electrolytic cell whereby it is converted to an oxalate salt. The electrolytic dimerization of carbon dioxide to the oxalate salt is conducted in an electrolytic cell wherein the anode and cathode compartments are separated by a porous membrane. The catholyte comprises a nonaqueous solvent having dissolved therein tetraethylammonium or tetrabutylammonium salts of perchloric acid, hydrobromic acid, hydriodic acid or p-toluenesulfonic acid. Coulombic yields as high as 75 percent are obtained where the catholyte is used as the anolyte while yields of sodium oxalate as high as 97 percent are obtained when the anolyte is an aqueous solution of sodium chloride.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Broadly, my invention relates to the production of oxalic acid by an electrolytic means. More specifically, my invention is directed to the preparation of an oxalate salt by means of the electrochemical dimerization of carbon dioxide followed by the acidification of the oxalate to produce oxalic acid. More particularly, my invention relates to the improvement of producing an oxalate salt in an electrolytic cell wherein the cell has an anode compartment and a cathode compartment separated by a cationic exchange porous membrane. The improvement comprises:

a. introducing carbon dioxide into the cathode compartment of the electrolytic cell to contact therein a catholyte comprising a non-aqueous liquid solvent and a C,-C alkyl quaternary ammonium salt soluble in said solvent,

b. applying a voltage to the electrodes of said cell to pass a direct current through said cell thereby forming an oxalate salt, and

c. recovering the oxalate salt from the catholyte.

The oxalate salt may be converted to oxalic acid by contacting the salt with an acid to produce a salt of the acid and oxalic acid.

The electrolytic cell employed may embody some of the various mechanical designs known heretofore. In general, the cell will comprise two compartments separated by a porous membrane to prevent the admixing of the electrolytes. Mechanical agitation of the catholyte or circulation of the catholyte through the cathode compartment is not essential but is preferred. An electrode is located in each compartment. Heating and cooling should be provided to maintain the operating temperature within the desired limits. The anode chamber may be constructed from the same materials used in electrolytic chlorine cells such as carbon steel. The cathode compartment may be constructed from carbon steel. Interior surfaces of the anode and cathode compartments may be lined with non-metallic materials such as polyvinylchloride or nylon.

The electrodes may be constructed from a variety of materials. The cathode material should have a high hydrogen overvoltage. Carbon, platinum, tin or zinc cathodes result in the generation of a gas and are not preferred. Copper or lead amalgamated cathodes as well as mercury, lead, or stainless steel cathodes produce the desired results. Because of corrosion problems in the anode compartment, inert or corrosion resistant anodes are employed. Generally, carbon or graphite anodes are preferred.

The anode and cathode compartments are separated by a membrane which prevents the anolyte and catholyte from mixing while being sufficiently porous to permit the passage of cations. In general, porous membranes constructed of sintered glass and cation exchange resin are useful while cation exchange resin is preferred.

The design of the electrolytic cell may be widely varied. For example, each half cell may be separately constructed and joined together with the porous membrane located at the junction. In one embodiment the cell may comprise a single compartment divided by a wall, a portion or all of which may constitute the porous membrane. In another design the half-cell compartments may comprise concentrically positioned chambers with the porous membrane located in the wall of the inner chamber.

Provisions must be made in the cell for the introduction of carbon dioxide in gaseous form into the cathode compartment, preferably within the vicinity of the cathode. One means to accomplish this is to provide a fritted disc or sparger in the compartment to introduce the carbon dioxide into the catholyte as a series of small bubbles. Improved contact between the gaseous carbon dioxide and the catholyte may be provided through mixing means located in the cathode compartment. Another means to introduce carbon dioxide into the cathode compartment is to add it to a circulating stream of catholyte at a point external to the cell. The circulating stream also provides a means of mixing the contents of the cathode compartment.

The desired reaction is conducted by employing a catholyte comprising a quaternary ammonium salt as an electrolyte dissolved in a non-aqueous solvent. Catholyte solvents which may be usefully employed include N,N-dimethyl-formamide (DMF), dimethylacetamide (DMA), dimethyl sulfoxide (DM- SO) and hexamethylphosphoramide (HMPA) with N,N-dimethylformamide being preferred. The electrolyte is dissolved in the solvent and is a quaternary ammonium salt. Useful electrolytes include the C -C alkyl quaternary ammonium salts soluble in the selected solvent, particularly the C -C alkyl quaternary ammonium salts. Preferred electrolytes include the tetraethylammonium, tetrapropylammonium and tetrabutylammonium salts of perchloric acid,

hydrobromic acid, hydriodic acid and p-toluenesulfonic acid with tetraethylammonium perchlorate (TEPC), tetraethylammonium bromide (TEBr), tetrabutylammonium perchlorate (TBPC), tetrabutylammonium iodide (TBI) and tetraethylammonium p-toluenesulfonate being especially preferred and tetraethylammonium perchlorate being more especially preferred.

Although an oxalate salt, which may be converted to oxalic acid, is produced when the same electrolyte is employed as the anolyte and the catholyte, decomposition of the electrolyte often occurs in the anode compartment and this mode of operation is not preferred. I have found that aqueous salt solutions may be usefully employed as the anolyte while permitting coulombic yields as high as 97 percent of the oxalate salt. Aqueous solutions of such salts as sodium chloride, sodium hydroxide and sodium bicarbonate have been usefully employed in the anode compartment. I have found that sodium salts are preferred because with the migration of the sodium ion to the cathode compartment the resultant sodium oxalate is readily converted to oxalic acid following its recovery from the compartment.

The broad and preferred ranges of the operating conditions for the electrolytic cell used in my invention are set forth in Table l below.

TABLE I OPERATING CONDITION BROAD PREFERRED Applied voltage, volts 1 to 50 5 to 20 Cathodic potential, volts l.75 to -3.00 l.80 to -2.30 (vs. a sat'd calomel reference electrode) Current density, milliamps/ l to 250 3 to 80 cm Temperature, C. 30 to 150 20 to 60 The process may be conducted as follows: After introducing the catholyte and the anolyte into their respective compartments and deaerating the catholyte with a purge of inert gas, such as nitrogen, carbon dioxide is bubbled through the catholyte. To initiate the dimerization of CO, and produce the oxalate, voltage is supplied to the electrodes to initiate the current flow which is continued for the duration of the run. 5 Preferably a magnetic bar is employed to maintain agitation within the catholyte compartment. When one of the quaternary ammonium salts is employed as a solute in both the anolyte and catholyte compartments, the oxalate produced is a quaternary ammonium oxalate which may be recovered from the catholyte by causing it to precipitate by the addition of diethyl ether. When an aqueous solution of sodium salt comprises the anolyte, sodium oxalate is produced in the catholyte as a white precipitate and may be recovered by filtration of the catholyte. Washing of the oxalate with an organic solvent, such as acetone, is followed by a drying step. The sodium oxalate is often recovered from the catholyte admixed with other salts, such as sodium bicarbonate. The oxalate may be recovered therefrom by combining the mixed salts with a mixture of aqueous acetic acid and acetone. The white suspension of sodium oxalate may be recovered by filtration and then acidified with an acid, such as hydrochloric acid to produce oxalic acid and sodium chloride. Addition of acetone results in sodium chloride crystals and a solution of oxalic acid in acetone. The sodium chloride crystals may be removed by filtration and the filtrate evaporated to dryness to yield oxalic acid. Altemately, the mixed salts may be mixed with hydrochloric acid followed by the addition of acetone to produce the oxalic acid.

Coulombic yields were determined as follows:

Percent coulombic yield:

The following examples exemplify the invention.

EXAMPLE I A series of runs demonstrating my invention were conducted in an electrolytic cell having two compartments separated by a cationic porous membrane consisting of cationic exchange resin. The cathode compartment was equipped with a lead electrode having a cross sectional area of 9.7 cm, a gas inlet tube and a magnetic stirring bar. The anode compartment was equipped with a carbon electrode having a cross sectional area of 12 cm. In this series of runs approximately 100 mililiters of the catholyte and 100 milliliters of the anolyte were added to the appropriate compartment. The catholyte was deaerated with a nitrogen stream for approximately five minutes and then carbon dioxide was introduced into the cathode compartment. A voltage was applied to the electrodes establishing a current flow through the electrolytes and continued throughout the period of the run. Upon the completion of each test run, the oxalate salt, in the form of tetraethylammonium oxalate was recovered by addition of diethyl ether.

Runs 6 to 14 show that higher yields of oxalate are obtained when an aqueous solution of sodium salt serves as the anolyte with yields as high as 97.8 percent being obtained.

EXAMPLE III In another series of runs conducted in the same manner as those of Example I, a number of cathodes were investigated. Although the catholyte in all in- IABLE II Coulombic Applied Maximum Run Current yield of Oatholyte and voltage temp. time density (CzO4=) Run No. anolyte 1 (volts) 0.) (hr.) (ma/cm?) (percent) 1 0.3 M TEPC/DMSO 33 13 3.0 75.6 20 6 4. 5 76. 6 30 31 6 2. 5 57. 9 20 33 20% 5. 0 73. 3 20 32 23 3. 9 58. 3

1 The abbreviations describe the following chemicals: TEPC-Tetraethylammonium perchlorate; DMSO- Dimethyl sulfoxide; DMA-Dimethylacetamide; HMPA-Hexamethylphosphoraniide; DMFN,N-dimethylde; TEBr-Tetraethylammonlum bromide.

These results show that the solvents tested gave favorable results.

EXAMPLE ii in another series of runs conducted in the same .manner as those of Example I, a number of an'olytes were investigated. In most of these runs the catholyte TABLE IV Coulomble Applied Maximum ltun Current yield of voltage temp. time density ((1204 Cathode (lutlmlyte (volts) 0.) (luz) (uuL/cmfl) (percent) Sut'd'llCi'Il/llitll 10 20.5 3 7.0 83.) I'll... ".3 M 'Il'ZII IDMI" 10 27.5 4 .l. 3 iiSL l il.lit, minimum... (1.3 M 'IiClC/llltil i0 27 5 MAJ 56.2 UlLllt: Hermann... Huttl 'lElV/DM l". 1U .50. 6 5 l2. 5 70.1 83..."... Sut'd 'iillU/UMl" 1O 2i? 3 13.0 50.8 Zn Hui/(i'ilClC/DMF l0 4 3 12.2! 58.2

was TEPC and DMF while in Runs 12 to 14 different EXAMPLE lV combinations of catholyte were studied. In all of these runs, Runs 6 to 14, the anolyte consisted of aqueous solutions of sodium salts which resulted in the production of sodium oxalate. The descriptions of the electrolytes, the operating conditions and the percent coulombic yield of sodium oxalate are set forth in Table III below.

A variety of electrolytes in the same solvent were investigated in Runs 21-25 which were conducted in the same manner as those of Example I. In all these runs the cathode was lead, the anode was graphite and the anolyte was a saturated aqueous solution of sodium TABLE III Coulombie Applied M axlmum Run Current yield of voltage temp. time density (C104=) Run No Catholyte and anolyte 1 (volts) 0.) (i112) (ma/cm?) (percent) 0.3M'IEPC/DMF fia p i i gg 15 14 8 I l a 7 8 "ls'am NuCl/HgO III} 15 33 0 7 1(l oi fi i do ()1 1% l9. 5 811! 11 3 l .55 7n. 2% 50.0 01.8

. r F H 12 Egg} g g/ 1 s5 15.0 7.9 P r t 1s gg fi ggg gfis a.) .4 z 25. a s3. 14 Nam/H20 a5 63. s 2 20. 8 s7. 5

See footnote at the end of Table II.

chloride. Operating conditions, the results and other details of Runs 21-25 are set forth in Table V below.

dimethylformamide, dimethylacetamide, dimethyl sulfoxide and hexamethylphosphoramide and a 1 The abbreviations describe the following materials: TBI-Tetrabutylammonium iodide; TEPC-Tetraethylammonium perchlorate; TBPCTetrabutylammonium perchlorate; TEBr-Tetraethylammonlum bromide; TEP'ISTetraethylammonium p-tolucnesulionate; DMF-N,N-dimethyllormamide.

From the results of Runs 21-25 it appears that the tetrabutylammonium salts give slightly higher coloumbic yields than the tetraethylammonium salts although all the electrolytes tested gave good results.

EXAMPLE V Typically, the sodium oxalate formed in the cathode compartment as in Examples ll-IV was recovered, when desired, by filtering the catholyte with suction. The resulting white solid was washed with acetone and dried in a vacuum oven under partial vacuum. The dried product consisted of sodium oxalate admixed with sodium bicarbonate. The mixed salt was treated with an aqueous solution of acetic acid and then with acetone. Filtering out the white precipitate yielded sodium oxalate which was collected and dried. The white product was then mixed with concentrated hydrochloric acid and the resultant slurry treated with acetone and filtered under suction. The white solid recovered was washed with acetone, dried and identified as sodium chloride. Evaporation of the filtrate to dryness yielded oxalic acid. The yield of oxalic acid was quantative, i.e., 100 percent.

The above examples demonstrate the process of our invention whereby oxalate salts are produced electrolytically and recovered and acidified to produce oxalic acid.

Obviously, many modifications and variations of the invention, as hereinbefore set forth, may be made without departing from the spirit and scope of this invention. Therefore, only such limitations should be imposed as are indicated in the claims set forth below.

I claim:

1. In a process for preparing oxalic acid wherein an oxalate salt is prepared and acidified to produce oxalic acid, the improvement of producing an oxalate salt in an electrolytic cell, said cell having an anode compartment and a cathode compartment separated by a cationic exchange porous membrane, which comprises:

a. introducing carbon dioxide into the cathode compartment of the electrolytic cell to contact therein a catholyte comprising a non-aqueous liquid solvent selected from the group consisting of N,N-

C,-C alkyl quaternary ammonium salt soluble in said solvent,

b. applying a voltage to the electrodes of said cell to pass a direct current through said cell thereby forming an oxlate salt, and

c. recovering the oxalate salt from the catholyte.

2. A process according to claim 1 wherein the quaternary ammonium salt is a C,-C alkyl quaternary ammonium salt.

3. A process according to claim 2 wherein the quaternary ammonium salt is selected from the group consisting of the tetraethylammonium tetraprop'ylammonlum and tetrabutylammonium salts of perc loric acid, hydrobromic acid, hydriodic acid and ptoluenesulfonic acid.

4. A process according to claim 3 wherein the quaternary ammonium salt is selected from the group consisting of tetraethylammonium perchlorate, tetraethylammonium bromide, tetrabutylammonium perchlorate, tetrabutylammonium iodide and tetraethylammonium p-toluenesulfonate.

5. A process according to claim 1 wherein the anolyte is selected from the group consisting of the catholyte, aqueous NaCl, aqueous NaOH and aqueous NaHCO;.

6. A process according to claim 1 wherein the applied voltage is between 1 and 50 volts and the cathodic potential, versus a saturated calomel reference electrode, is between 1 .75 and 3.00 volts.

7. A process according to claim 1 wherein the current density is between about i and about 250 milliamps per square centimeter of electrode area.

8. A process according to claim 1 wherein the cathode materials are selected from the group consisting of copper amalgam, lead amalgam, lead, mercury and stainless steel.

9. A process according to claim 1 wherein the anode material is graphite.

10. A process according to claim 1 including the following additional steps:

d. contacting the oxalate salt with an acid thereby producing a salt of the acid and oxalic acid, and

e. recovering oxalic acid as a product. 

1. In a process for preparing oxalic acid wherein an oxalate salt is prepared and acidified to produce oxalic acid, the improvement of producing an oxalate salt in an electrolytic cell, said cell having an anode compartment and a cathode compartment separated by a cationic exchange porous membrane, which comprises: a. introducing carbon dioxide into the cathode compartment of the electrolytic cell to contact therein a catholyte comprising a non-aqueous liquid solvent selected from the group consisting of N,N-dimethylformamide, dimethylacetamide, dimethyl sulfoxide and hexamethylphosphoramide and a C1-C5 alkyl quaternary ammonium salt soluble in said solvent, b. applying a voltage to the electrodes of said cell to pass a direct current through said cell thereby forming an oxlate salt, and c. recovering the oxalate salt from the catholyte.
 2. A process according to claim 1 wherein the quaternary ammonium salt is a C2-C4 alkyl quaternary ammonium salt.
 3. A process according to claim 2 wherein the quaternary ammonium salt is selected from the group consisting of the tetraethylammonium, tetrapropylammonium and tetrabutylammonium salts of perchloric acid, hydrobromic acid, hydriodic acid and p-toluenesulfonic acid.
 4. A process according to claim 3 wherein the quaternary ammonium salt is selected from the group consisting of tetraethylammonium perchlorate, tetraethylammonium bromide, tetrabutylammonium perchlorate, tetrabutylammonium iodide and tetraethylammonium p-toluenesulfonate.
 5. A process according to claim 1 wherein the anolyte is selected from the group consisting of the catholyte, aqueous NaCl, aqueous NaOH and aqueous NaHCO3.
 6. A process according to claim 1 wherein the applied voltage is between 1 and 50 volts and the cathodic potential, versus a saturated calomel reference electrode, is between -1.75 and -3.00 volts.
 7. A process according to claim 1 wherein the current density is between about 1 and about 250 milliamps per square centimeter of electrode area.
 8. A process according to claim 1 wherein the cathode materials are selected from the group consisting of copper amalgam, lead amalgam, lead, mercury and stainless steel.
 9. A process according to claim 1 wherein the anode material is graphite. 